To reduce the storage volume, gaseous fluids can be stored in liquefied form at cryogenic temperatures when employed for example as fuel for internal combustion engines. A gaseous fluid including a gaseous fuel is defined as any fluid that is in a gas state at standard temperature and pressure which is defined herein as 1 atmosphere and between 20 and 25 degrees Celsius. Gaseous fuel is normally stored near its boiling point in a storage vessel. For example, for methane at a storage pressure of about 1 atmosphere it can be stored in liquefied form at a temperature of about −161 degrees Celsius. Natural gas is a mixture of gasses with methane typically comprising the largest fraction; storage temperature can vary, but is normally close to that of methane. From the storage vessel, the gaseous fluid can be supplied in either a liquefied or a vapor state to a heater where the temperature of the gaseous fluid is increased for delivery at a desired pressure and temperature for use for example for injecting into an engine.
In known systems, a heater vaporizes liquefied gaseous fuel when the gaseous fuel is delivered to an engine. The heater can be a heat exchanger that transfers heat energy from warmer engine coolant to the colder liquefied gas. After vaporization in a vaporizer, a fuel injection system receives vaporized gaseous fuel and introduces it, either directly or indirectly, to one or more combustion chambers in an engine. As used herein, vaporizing refers to at least increasing the enthalpy (that is, temperature) of the gaseous fluid as it passes through the heater, and depending upon the pressure and the temperature of the gaseous fluid it can also refer to changing the state of the gaseous fluid from a liquefied or supercritical state to the gas state. While liquefied natural gas (LNG) is an exemplary liquefied gaseous fluid, other gaseous fluids are equally applicable to the technique described herein.
It is important to control the vapor pressure of the gaseous fluid within its cryogenic supply tank as well as the vapor pressure and temperature at which it is supplied to certain use device systems including gaseous fueled engine systems. The pressure and temperature must be above predetermined minimum values such that gaseous fluid can be delivered at a needed pressure to a use device while protecting components from excessively cold temperatures that could cause component failure. Additionally it is important to keep pressures within the tank below pressures that would cause venting to atmosphere and/or in more extreme cases failure of the tank. This pressure and temperature control is managed through a fluid supply circuit in fluid communication with the tank such that the pressure and temperature is maintained between predetermined system thresholds. Fluid can be delivered from the tank to a vaporizer from either a conduit generally in fluid communication with the liquefied portion or a conduit generally in fluid communication with the vapor portion of the tank, or a combination thereof, through actively controlled valves on the two separate fluid delivery lines.
Prior art systems used for metering gaseous fuel to an engine are typically mechanical in nature using what is termed in the industry as an “economizer” which is a mechanical valve with a fixed pressure setting. When the tank pressure is higher than a set pressure, the economizer sends vapor from the tank to the vaporizer and when tank pressure is below the set pressure the economizer valve closes and liquid is delivered to the vaporizer. Cryogenic fuel tanks typically have high tank pressure after filling up the tank at refueling or after sitting for multiple days without use. The economizer uses tank pressure when tank pressure is high to prevent any vapor being vented out of the tank. However mechanical systems such as ones that employ an economizer valve have no way of reacting to varying use device demands; these systems are designed to blindly consume the vapor from the tank first only when the tank pressure has fallen below a set point, such systems will switch over to liquid fuel delivery.
An electronically controlled pressure management system is described in U.S. Pat. No. 6,058,713 by Bowen et al. which discloses a gaseous fuel control delivery system that delivers fuel from a cryogenic storage tank as a function of engine demand while maintaining a safe pressure within the fuel tank by using a CPU for determining when to open a separate liquid fuel delivery valve or a vapor fuel delivery valve based on engine demand. However while this system takes into account engine demand it has been found that engine operation and fluid handling of the cryogenic fuel are still managed inefficiently by such systems because pressure set points are still fixed; for example, this results in vapor pressure being reduced at times when it would be advantageous to keep vapor pressure higher.
There is a need for an improved control strategy for managing the vapor pressure of fueling systems using cryogenic fluids in the storing and supplying of fuel to gaseous fueled engine systems. A solution is disclosed herein which employs pressure sensors, electronically controlled solenoid valves and a programmed electronic controller to form an intelligent predictive system for managing the pressure, and additionally the temperature in some exemplary systems, of the cryogenic fluid system and for determining fluid delivery modes to a use device such as an engine for an improved efficient system operation. The vapor pressure is used as the pump of the system and maintaining the desired pressure and temperature is central to having a robust, responsive and efficient fluid delivery system while avoiding the problems of prior art systems.